I am doing an estimation of erosion with ArcGIS. I use the UniversalSoilLossEquation (USLE) for this.
Now, i want to calculate the L-factor of this equation. I need the FlowLength for this. My problem begins here:
I calculated the Slope of the DEM –> Slope_DEM
Now i want to do the following: I want to calculate the changes in slope angles between cells and create "NoData" if the change is more than 30%.
Example: If one cell has a slope angle of 50° and the next cell has an angle of 25°, I want to create NoData cause the change of slope angle is more than 30% (here it is 50%). After this the flowlength resets
How can I realize this in ArcGIS?
Here is something I found in literature which describes what I want to do very well:
"This particular problems is addressed by the cutoff slope angle which
is a user-input value. The cutoff slope angle is defined as the change
in slope angle from one cell to the next along the flowdirection. This
value ranges from 0 to 1 for all areas where the slope angle decreases
from one cell to the next (if the slope angle increases, there will
definitely be no deposition). Therefore, the user input value will
range from 0 – 1 and be dependent upon the amount of sediment carried
by overland flow. For example, an input value of zero will cause the
slope length to reset every time there is a decrease in slope angle.
An input value of one will cause the slope length to never reset. In
an ideal world, this value would be set by an expert familiar with the
particular area in question. However, this is not always feasible. The
literature(Griffin,et al., 1988; Wilson, 1986) suggests references
that a value closer to 0.5 (slopedecreasing by 50% or greater) is
appropriate.
(source: http://onlinegeographer.com/slope/hickey_slope_length.pdf)
Best Answer
The original USLE assumed little slope curvature and no deposition. To account for flow convergence in complex terrain, modifications were made to the LS factor with an equation that incorporates flow accumulation. Because they are now used interchangeably, when you see reference to slope-length most likely it is in reference to the LS factor, as calculated below.
To calculate the LS factor for the RUSLE equation, first calculate flow accumulation (facc) and slope in degrees (slp). Then a bit of map algebra in the raster calculator yields LS.
Power(facc * cell resolution / 22.1, 0.4) * Power(Sin(slp * 0.01745) / 0.09, 1.4) * 1.4